Adaptive control of reformer inlet oxygen concentration

ABSTRACT

A vehicle on-board computer method for controlling a diesel engine intake air throttle valve during LNT regeneration. The method comprises setting a software limit on the throttle valve position. The limit is a dynamic value affected by the engine operating state. A variable relating to the exhaust oxygen concentration is controlled to approach a setpoint by adjusting the intake air throttle valve position subject to the limit. In one embodiment, the valve position limit is based in part on an exhaust oxygen content measured while the throttle valve is fully open. In another embodiment, the valve position limit is adjusted based on the setpoint, whereby greater throttling is allowed when exhaust oxygen content is targeted to a higher value. In a further embodiment, the setpoint is adjusted based on the engine load. The method allows extensive intake air throttling without causing instability in the engine.

FIELD OF THE INVENTION

The present invention relates to pollution control devices for diesel engines.

BACKGROUND

NO_(x) and particulate matter (soot) emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NO_(x) and particulate matter (soot) emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations. Diesel particulate filters (DPFs) have been proposed for controlling particulate matter emissions. A number of different solutions have been proposed for controlling NOx emissions.

In gasoline powered vehicles that use stoichiometric fuel-air mixtures, NO_(x) emissions can be controlled using three-way catalysts. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.

One set of approaches for controlling NOx emissions from diesel-powered vehicles involves limiting the creation of pollutants. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful in reducing NOx emissions, but these techniques alone are not sufficient. Another set of approaches involves removing NOx from the vehicle exhaust. These approaches include the use of lean-burn NO_(x) catalysts, selective catalytic reduction (SCR), and lean NO_(x) traps (LNTs).

Lean-burn NOx catalysts promote the reduction of NO_(x) under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NO_(x) catalyst that has the required activity, durability, and operating temperature range. Lean-burn NO_(x) catalysts also tend to be hydrothermally unstable. A noticeable loss of activity occurs after relatively little use. Lean-burn NOx catalysts typically employ a zeolite wash coat, which is thought to provide a reducing microenvironment. The introduction of a reductant, such as diesel fuel, into the exhaust is generally required and introduces a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low.

SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.

To clarify the state of a sometime ambiguous nomenclature, it should be noted that in the exhaust aftertreatment art, the terms “SCR catalyst” and “lean NOx catalyst” are occasionally used interchangeably. Where the term “SCR” is used to refer just to ammonia-SCR, as it often is, SCR is a special case of lean NOx catalysis. Commonly when both types of catalysts are discussed in one reference, SCR is used with reference to ammonia-SCR and lean NOx catalysis is used with reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons.

LNTs are devices that adsorb NOx under lean exhaust conditions and reduce and release the adsorbed NOx under rich exhaust conditions. A LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO₃ and the catalyst is typically a combination of precious metals, such as Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. In a reducing environment, the catalyst activates reactions by which adsorbed NOx is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to remove accumulated NOx and thereby regenerate (denitrate) the LNT.

Creating a reducing environment for LNT regeneration involves providing a reductant to the exhaust. Except where the engine can be run stoichiometric or rich, a portion of the reductant is consumed by reactions that remove oxygen from the exhaust. The amount of oxygen to be removed in this manner and the amount of reductant thus consumed can be reduced in various ways. If the engine is equipped with an intake air throttle, the throttle can be used. However, at least in the case of a diesel engine, it is generally necessary to eliminate some of the oxygen in the exhaust by combustion or reforming reactions with reductant that is injected into the exhaust.

The reactions between reductant and oxygen can take place in the LNT, but it is generally preferred for the reactions to occur in a catalyst upstream of the LNT, whereby the heat of reaction can be dissipated to some extent before transferring to the LNT. Temperature excursions can have an undesirable effect on the LNT.

Reductant can be injected into the exhaust by the engine fuel injectors or separate injection devices. For example, the engine can inject extra fuel into the exhaust within one or more cylinders prior to expelling the exhaust. Alternatively, or in addition, reductant can be injected into the exhaust downstream of the engine.

U.S. Pat. Pub. No. 2004/0050037 (hereinafter “the '037 publication”) describes an exhaust treatment system with a fuel reformer placed in the exhaust line upstream of a LNT. The reformer includes both oxidation and reforming catalysts. The reformer both removes excess oxygen and converts the diesel fuel reductant into more reactive reformate.

The operation of an inline reformer, whether a partial oxidation reformer, an auto thermal reformer, or a steam reformer, can be modeled in terms of the following three reactions:

0.684CH_(1.85)+O₂→0.684CO₂+0.632H₂O  (1)

0.316CH_(1.85)+0.316H₂O→0.316CO+0.608H₂  (2)

0.316CO+0.316H₂O→0.316CO₂+0.316H₂  (3)

wherein CH_(1.85) represents an exemplary reductant, such as diesel fuel, with a 1.85 ratio between carbon and hydrogen. Reaction (1) is exothermic complete combustion by which oxygen is consumed. Reaction (2) is endothermic steam reforming. Reaction (3) is the water gas shift reaction, which is comparatively thermal neutral and is not of great importance in the present disclosure, as both CO and H₂ are effective for regeneration.

The inline reformer of the '037 publication is designed to be rapidly heated and to then catalyze steam reforming. Temperatures from about 500 to about 700° C. are said to be required for effective reformate production by this reformer. These temperatures are substantially higher than typical diesel exhaust temperatures. The reformer is heated by injecting fuel at a rate that leaves the exhaust lean, whereby Reaction (1) takes place. After warm up, the fuel injection rate is increased to provide a rich exhaust. Depending on such factors as the exhaust oxygen concentration, the fuel injection rate, and the exhaust temperature, the reformer tends to either heat or cool as reformate is produced. Reformate is an effective reductant for LNT denitration.

During denitration, much of the adsorbed NOx is reduced to N₂, but a portion of the adsorbed NOx is released without having been reduced and another portion of the adsorbed NOx is deeply reduced to ammonia. The NOx release occurs primarily at the beginning of the regeneration. The ammonia production has generally been observed towards the end of the regeneration.

U.S. Pat. No. 6,732,507 proposes a system in which a SCR catalyst is configured downstream of the LNT in order to utilize the ammonia released during denitration. The LNT is provided with more reductant over the course of a regeneration than required to remove the accumulated NOx in order to facilitate ammonia production. The ammonia is utilized to reduce NOx slipping past the LNT and thereby improves conversion efficiency over a stand-alone LNT.

U.S. Pat. Pub. No. 2004/0076565 (hereinafter “the '565 publication”) also describes hybrid systems combining LNT and SCR catalysts. In order to increase ammonia production, it is proposed to reduce the rhodium loading of the LNT. In order to reduce the NOx release at the beginning of the regeneration, it is proposed to eliminate oxygen storage capacity from the LNT.

In addition to accumulating NOx, LNTs accumulate SOx. SOx is the combustion product of sulfur present in ordinarily fuel. Even with reduced sulfur fuels, the amount of SOx produced by combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere. The temperature of the exhaust can be elevated by engine measures, particularly in the case of a lean-burn gasoline engine, however, at least in the case of a diesel engine, it is often necessary to provide additional heat. Typically, this heat can be provided through the same types of reactions as used to remove excess oxygen from the exhaust. Once the LNT is sufficiently heated, the exhaust is made rich by measures like those used for LNT denitration.

Diesel particulate filters must also be regenerated. Regeneration of a DPF is to remove accumulated soot. Two general approaches are continuous and intermittent regeneration. In continuous regeneration, a catalyst is provided upstream of the DPF to convert NO to NO₂. NO₂ can oxidize soot at typical diesel exhaust temperatures and thereby effectuate continuous regeneration. A disadvantage of this approach is that it requires a large amount of expensive catalyst.

Intermittent regeneration involves heating the DPF to a temperature at which soot combustion is self-sustaining in a lean environment. Typically this is a temperature from about 400 to about 600° C., depending in part on what type of catalyst coating has been applied to the DPF to lower the soot ignition temperature. A challenge in using this approach is that soot combustion tends to be non-uniform and high local temperatures can lead to degradation of the DPF.

In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and is practical for reducing NOx emissions from diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations.

SUMMARY

One of the inventor's concepts relates to a vehicle on-board computer method for controlling a position of a diesel engine intake air throttle valve. Typically the throttle is controlled during an exhaust system regeneration process, such as LNT denitration or DPF soot combustion. The method comprises setting a software limit on the throttle valve position. The limit is a value that changes in a manner relating to one or more of engine speed and engine load. The limit is used in a control algorithm in which a variable relating to the exhaust oxygen content, such as an exhaust oxygen concentration, an exhaust oxygen flow rate, or an exhaust air-fuel ratio is controlled to approach a setpoint value by adjusting the intake air throttle valve position. In one embodiment, the valve position limit is based in part on an exhaust oxygen content that is measured while the throttle valve position is fully open. In another embodiment, the valve position limit is adjusted based on the setpoint, whereby greater throttling is allowed when exhaust oxygen content is targeted to a higher value. In a further embodiment, the setpoint is adjusted based on the engine load, whereby targets for lowering the exhaust oxygen content are less aggressive when the engine load is high. The method allows extensive intake air throttling and associated benefits for regeneration processes without causing instability in the engine operation. The method is particularly useful when the intake air throttle and controller are added to an engine that was not designed to operate with an intake air throttle.

Another concept of the inventor relates to a method of operating a power generation system. The method comprises operating a diesel engine to produce an exhaust, which is passed through a fuel reformer and a LNT. To regenerate the LNT, the exhaust is made rich, whereby the fuel reformer produces reformate-containing exhaust. As part of the regeneration sequence, a throttle valve position limit is set based on a state of the diesel engine. The engine intake air throttle valve position is controlled to reduce the exhaust oxygen and drive it towards a setpoint while not exceeding the throttle valve position limit.

A further concept of the inventor relates to a power generation system comprising a diesel engine having an intake air throttle valve, an exhaust treatment system, and a controller. The controller is operative to regenerate an exhaust aftertreatment device by measures including controlling the position of the engine intake air throttle valve. The controller is configured to control the engine intake air throttle valve position during the regeneration to approach the exhaust oxygen content to a setpoint. The controller is configured not to adjust the valve position beyond a limit that is set dynamically by the controller. Preferably, the power generation system can be assembled by adding an exhaust treatment system to an engine designed independently of the intake air throttle and the exhaust treatment system.

The primary purpose of this summary has been to present certain of the inventors' concepts in a simplified form to facilitate understanding of the more detailed description that follows. This summary is not a comprehensive description of every one of the inventor's concepts or every combination of the inventor's concepts that can be considered “invention”. Other concepts of the inventor will be conveyed to one of ordinary skill in the art by the following detailed description together with the drawings. The specifics disclosed herein may be generalized, narrowed, and combined in various ways with the ultimate statement of what the inventor claim as his invention being reserved for the claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an exemplary control architecture conceived by the inventor.

FIG. 2 is a schematic illustration of an exemplary power generation system in which the inventor's concepts may be implemented.

DETAILED DESCRIPTION

One of the inventor's concepts relates to a method of controlling a diesel engine intake air throttle valve in a power generation system. The method is generally applied to reduce the exhaust oxygen concentration during an exhaust aftertreatment device regeneration event, such as LNT or DPF regeneration. The goal of the method is to limit the exhaust oxygen concentration without destabilizing the operation of the engine. Preferably, the intake air throttling does not cause the engine to exceed a smoke limit and does not have any effect noticeable to an operator of a vehicle driven by the power generation system. The method is particularly useful when the aftertreatment system, including the intake air throttle valve, is an add-on to an existing engine design.

According to the inventor's concept, a software limit is dynamically placed on an intake air throttle valve position. The software limit is a restriction in the degree to which the throttle valve can be closed; whereby respecting the limit restricts the degree to which the throttle valve can be closed beyond any limit inherent in the throttle valve hardware. Dynamically does not mean continuously changing, but does mean the limit is reset over the course of operating the power generation system and is set based on current operating conditions rather than a predetermined value.

The software limit on the intake air throttle valve position may be set at any appropriate time during operation of the power generation system. In one embodiment, the limit is set at the onset of intake air throttling. For example, the transition from an un-throttled state to a partially throttled state may be used to trigger setting the intake air throttle valve position limit. In another example, the limit is set at the onset of an exhaust aftertreatment device regeneration event, such as a LNT denitration, a LNT desulfation, or a DPF regeneration.

The software limit is set to a value depending on the engine operating state. The engine operating state is characterized primarily by engine speed and load, but may involve many other variables such as an EGR valve position, a turbocharger vane position, atmospheric pressure, ambient temperature, and so forth. In general, the value varies at least with engine speed and load, although these values need not be considered directly in setting the software limit. Instead, a value that is itself affected by engine speed and load, such as un-throttled exhaust oxygen concentration, may be referenced in setting the software limit.

According to another of the inventor's concepts, the software limit on the engine intake air throttle valve is set based on an exhaust oxygen concentration measured while the intake air throttle valve is fully open. The oxygen concentration can be measured just before or coinciding with the onset of an event, such as a LNT regeneration process, which involved intake air throttling. The measurement may therefore be triggered by a control signal initiating such an event.

The initial exhaust oxygen concentration provides a good indication of the extent to which the intake air throttle valve may be closed before disrupting operation of the engine. When the un-throttled exhaust oxygen concentration is high, a relatively high degree of throttling can be used. When the un-throttled exhaust oxygen concentration is low, a high degree of throttling is likely to be problematic. The limits to be used can be determined experimentally and encoded in a formula or table.

According to another of the inventor's concepts, the throttle valve position limit is set based in part on the target value (setpoint) for the exhaust oxygen content. An exhaust oxygen content can be characterized by an exhaust oxygen concentration, an exhaust oxygen flow rate, or an exhaust air/fuel ratio. When higher exhaust oxygen contents are targeted, there is less risk of destabilizing the engine operation and greater flexibility in setting the throttle valve position can be allowed; therefore, the throttle valve position limit is preferably set in such a way that a greater degree of throttling is permitted when higher exhaust contents are being targeted. In one embodiment, the throttle valve position limit is initially set based on the engine operating state and is then adjusted slightly based on the target exhaust oxygen concentration. A slight adjustment would be from about 1 to about 20% based on the throttle valve's range of motion.

Another method of avoiding engine instability during control of intake air throttling is to choose an appropriate setpoint for the exhaust oxygen content. According to a further concept of the inventor, an exhaust oxygen content setpoint is determined from or adjusted based on an engine operating condition, preferably the engine load. As engine load increases, throttling becomes more likely to destabilize engine operation. Accordingly, in an exemplary embodiment, a setpoint for exhaust oxygen content is determined in a manner whereby the setpoint's minimum value increases with engine load. If it is desirable to reduce the exhaust oxygen content as much as possible, the setpoint is placed at the minimum value. If the exhaust oxygen content may be targeted to a higher level, than the minimum value is simply a limit on the setpoint.

FIG. 1 is a schematic illustration of an exemplary control architecture 1 implementing several of the inventor's concepts. A feedback controller 3 is activated by a command 2 to initiate intake air throttling. Prior to receiving this command, the intake air throttle valve position 4 is expected to be fully open. The feedback controller 3 causes the throttle valve 4 to begin closing. The position of the throttle valve is monitored, and a position threshold indicator 5 recognizes that the intake throttle has begun to close and communicates this information to the limit scheduler 6. The limit scheduler 6 responds by taking a measurement of the exhaust oxygen concentration 7 from the exhaust oxygen concentration predictor 8. Based on this measurement, the limit scheduler 6 provides a base value for the intake air throttle position limit to the multiplier 9.

The desired oxygen concentration is set in block 10. This concentration may be set in part based on the current engine load. The desired oxygen concentration is supplied to the feedback controller 3 and the reference factor calculator 11. The reference factor calculator 11 provides an adjustment factor for the throttle position limit based on the desired oxygen concentration. The adjustment factor is multiplied with the base intake air throttle position limit in the multiplier 9. The product is the intake air throttle position limit, which is supplied to the feedback controller 3.

The feedback controller 3 commands adjustments in the intake air throttle position 4 to cause the oxygen concentration 7 as estimated by oxygen concentration predictor 8 to approach the desired oxygen concentration (setpoint) provided by block 10. The commands are limited so that the intake air throttle position 4 is never commanded to exceed the intake air throttle position limit provided by the multiplier 9.

The feedback controller 3 can implement and suitable control strategy. An exemplary control strategy is a combination of proportional, integral, and optionally differential control. Integral control involves integrating an error. In this case, the error is the difference between the predicted oxygen concentration provided by block 8 and the desired oxygen concentration provided by block 10. The integral is used as a weighting factor in a formula used to determine the commanded throttle position. According to another of the inventor's concepts, this integration is discontinued when the commanded throttle position reaches the throttle position limit. This feature is analogous to the anti-windup mechanism used when encountering a hardware limit in conventional PID control, the difference being that in the present case, the anti-windup feature is based on a software limit.

A measurement of the exhaust oxygen concentration 7 can be provided directly to the feed controller 3, but in this example the predictor 8 is used. The predictor 8 functions to stabilize the controller 3 by partially compensating for the time delay between changing the throttle position and receiving a response from a detector of the exhaust oxygen concentration. The prediction may be a simple extrapolation, however, a preferred prediction uses a weighted average of the measured exhaust oxygen concentration and a model prediction of the exhaust oxygen concentration. The model prediction is preferably made using a model having dynamics that are faster than the modeled system.

FIG. 2 provides an exemplary power generation system 20 in which the inventor's concepts can be implemented. The system 20 comprises an engine 21, a intake manifold 22 configured with a throttle valve 23, and an exhaust aftertreatment system 24. The exhaust aftertreatment system 24 includes a controller 25 and an exhaust line 26. Configured within the exhaust line 26 are a first fuel injector 27, diesel particulate filter (DPF) 28, a second fuel injector 29, a fuel reformer 30, a thermal mass 31, a lean NOx-trap (LNT) 32, an ammonia-SCR catalyst 33. The controller 25 receives data from several sources; include exhaust oxygen concentration sensor 34, temperature sensors 35 and 36 and NOx sensor 37. The controller 25 may be an engine control unit (ECU) that also controls the exhaust aftertreatment system 24 or may include several control units that collectively perform these functions.

The DPF 28 removes particulates from the exhaust. During lean operation (a lean phase), the LNT 32 adsorbs a portion of the NOx from the exhaust. The ammonia-SCR catalyst 33 may have ammonia stored from a previous regeneration of the LNT 32 (a rich phase). If the ammonia-SCR catalyst 33 contains stored ammonia, it removes a second portion of the NOx from the lean exhaust.

From time-to-time, the LNT 32 must be regenerated to remove accumulated NOx (denitrated). Denitration may involve heating the reformer 30 to a steam-reforming temperature and then injecting fuel using the fuel injector 29. Heating the reformer 30 generally involves injecting fuel at a rate that leaves the exhaust lean, whereby the fuel combusts in the reformer 30 generating heat. The fuel injection rate is then set to a level that makes the exhaust rich, whereby the reformer 30 uses the injected fuel to consume most of the oxygen from the exhaust while producing reformate. The reformate thus produced reduces NOx adsorbed in the LNT 32. Some of this NOx is reduced to NH₃, most of which is captured by the ammonia-SCR catalyst 33 and used to reduce NOx during a subsequent lean phase. The thermal mass 31 stores and slowly releases heat, preventing the fuel reformer 30 from excessively heating the LNT 32 at every regeneration.

In principle, the temperature of the reformer 30 can be controlled through the fueling rate. If the fueling rate is at least a stoichiometric rate, increasing the fueling rate will in principle increase the proportion of endothermic steam reforming, reaction (2), in comparison to exothermic combustion, reaction (1). In practice, due to the limited efficiency of the fuel reformer 30, the spatial segregation between where the reactions occur coupled with the limits on heat transfer rates, and the limited capacity of the LNT 32 to use reformate efficiently, the heating of the fuel reformer 30 cannot be effectively controlled for all exhaust oxygen concentrations just by changing the fuel injection rate. For exhaust oxygen concentrations above about 8%, the reformer 30 tends to heat excessively and may need to be shut down and allowed to cool before regeneration of the LNT 32 is completely regenerated. When steam reforming is not used, the problem of excessive heat generation may be even more acute. For these reasons, and to save fuel, it is highly desirable to reduce the exhaust oxygen concentration by throttling the engine intake air during LNT regeneration.

The time at which to regenerate the LNT 32 to remove accumulated NOx can be determined by any suitable method. Examples of methods of determining when to begin a regeneration include initiating a regeneration upon reaching a threshold in any of a NOx concentration in the exhaust, a total amount of NOx emissions per mile or brake horsepower-hour over a previous period or since the last regeneration, a total amount of engine out NOx since the last regeneration, an estimate of NOx loading in the LNT 32, and an estimate of adsorption capacity left in the LNT 32. Regeneration can be periodic or determined by feed forward or feedback control. Regeneration can also be opportunistic, being triggered by engine operating conditions that favor low fuel penalty regeneration or easily maintain a low exhaust oxygen concentration. A threshold for regeneration can be varied to give a trade off between urgency of the need to regenerate and favorability of the current conditions for regeneration.

The time at which to regenerate the LNT 32 can be determined by the controller 25, which generates a control signal that initiates the regeneration process. The control signal drives a series of events, which can include warming the fuel reformer 30, if it is of a type that requires warming, and then injecting fuel at a rate that makes the exhaust rich until the LNT 32 is regenerated to a satisfactory degree. The series of events may also include setting the intake air throttle position limit. Throttling need not begin before the rich phase in which reformate is generated, however, it may be desirably to begin throttling earlier, as the exhaust oxygen concentration responds relatively slowly to intake air throttling.

In addition to accumulating NOx, the LNT 32 accumulates SOx. SOx is the combustion product of sulfur present in ordinarily fuel. Even with reduced sulfur fuels, the amount of SOx produced by combustion is significant. SOx adsorbs more strongly than NOx and necessitates a more stringent, though less frequent, regeneration. Desulfation requires elevated temperatures as well as a reducing atmosphere. In the system 20, the LNT 32 can be heated through the reformer 30. The reformer 30 is provided with fuel at a rate that first heats the reformer 30 and then maintains the reformer 30 at an elevated temperature. The LNT 32 is then warmed by heat convection. Once the LNT 32 is sufficiently heated, the exhaust is made rich.

Desulfation is a much slower process than denitration. Steam reforming is even less effective during desulfation for balancing the heat produced while consuming oxygen from the exhaust than in denitration in that large concentrations of reformate cannot be utilized effectively during desulfation. Accordingly, it is even more desirable during desulfation to reduce the exhaust oxygen concentration by intake air throttling than in denitration. On the other hand, desulfation contributes less to the overall fuel cost than desulfation and there is the alternative of pulsing the fuel injection: periodically shutting the fuel reformer down to allow it to cool. The disadvantage of pulsing the fuel injection in this manner is that it can substantially increase the length and fuel penalty for a regeneration.

Whereas denitration take only a few seconds, desulfation typically requires a number of minutes. In denitration, it is generally sufficient to set the intake air throttle position limit once at the beginning of the regeneration event and forego adjustments based on changing engine operating conditions. In desulfation, it is preferred that the intake air throttle position limit be revised from time-to-time based on changing engine operating conditions.

From time-to-time, the DPF 28 must be regenerated to remove accumulated soot. Regeneration involves heating the DPF 28 to a temperature at which soot combustion is self-sustaining in a lean environment. Typically this is a temperature from about 400 to about 600° C., depending in part on what type of catalyst coating has been applied to the DPF 28 to lower the soot ignition temperature. The DPF 28 is heated by injecting fuel with the first fuel injector 27. The fuel can combust in the DPF 28 or an optional upstream oxidation catalyst. Soot combustion is exothermic. Once the DPF 28 has heated to a temperature at which the heat released by soot combustion is equal or greater than the rate of cooling by conduction, convection, and radiation at at least one point within the DPF 28, fuel injection can discontinue.

A problem with soot combustion is that is can heat the DPF 28 to excessive temperatures. Soot combustion tends to propagate through the DPF 28 along fronts. These fronts create local hot spots that can damage the DPF 28. To endure the high temperatures occurring at these hot spots, a typical DPF requires expensive substrate materials, such as the more expensive SiC in place of the less expensive cordierite. To avoid the need for such an expensive high temperature material, the system 20 limits the hot-spot temperatures in the DPF 28 by limiting the exhaust oxygen concentration. The exhaust oxygen concentration is limited by intake air throttling. As in desulfation, the intake air throttle position limit is generally set at the beginning of the regeneration, but is also typically adjusted over the course of the regeneration to respond to changing engine operating conditions.

When none of the aftertreatment devices is being regenerated, the engine intake air throttle valve 23 is generally fully open. The engine 21 generally operates best without any intake air throttling. Intake air throttling will generally reduce the fuel efficiency of the engine 21, although this reduced fuel efficiency is generally more than offset by the reduced fuel penalty of exhaust aftertreatment device regeneration when throttling is used during these regeneration events.

The methods of the inventor are particularly useful when the aftertreatment system 24 is a “bolt on” system, meaning that the engine 21 is designed and manufactured independently of at least some of the aftertreatment system components. The engine 21 may be designed and manufactured without an intake air throttle valve 23. In one embodiment, the engine 21 has an ECU that is programmed independently of the controller 25. The use of a “bolt on” aftertreatment system considerably eases the burden on an engine manufacturer wishing to produce a power generation system meeting new emission regulations. The engine can be deigned and manufactured using proven designs and the aftertreatment system added later to meet regulations.

While the engine 21 is preferably a compression ignition diesel engine, the various concepts of the inventor are applicable to power generation systems with lean-burn gasoline engines or any other type of engine that produces an oxygen rich, NOx-containing exhaust. For purposes of the present disclosure, NOx consists of NO and NO₂.

The power generation system 20 can have any suitable types of transmission. A transmission can be a conventional transmission such as a counter-shaft type mechanical transmission, but is preferably a CVT. A CVT can provide a much larger selection of operating points than a conventional transmission and generally also provides a broader range of torque multipliers. The range of available operating points can be used to control the exhaust conditions, such as the oxygen content. A given power demand can be met by a range of torque multiplier-engine speed combinations. A point in this range that gives acceptable engine performance while best meeting a control objective, such as minimum oxygen flow rate, can be selected.

In general, a CVT will also avoid or minimize interruptions in power transmission during shifting. Examples of CVT systems include hydrostatic transmissions; rolling contact traction drives; overrunning clutch designs; electrics; multispeed gear boxes with slipping clutches; and V-belt traction drives. A CVT may involve power splitting and may also include a multi-step transmission.

A preferred CVT provides a wide range of torque multiplication ratios, reduces the need for shifting in comparison to a conventional transmission, and subjects the CVT to only a fraction of the peak torque levels produced by the engine. This can be achieved using a step-down gear set to reduce the torque passing through the CVT. Torque from the CVT passes through a step-lip gear set that restores the torque. The CVT is further protected by splitting the torque from the engine, and recombining the torque in a planetary gear set. The planetary gear set mixes or combines a direct torque element transmitted from the engine through a stepped automatic transmission with a torque element from a CVT, such as a band-type CVT. The combination provides an overall CVT in which only a portion of the torque passes through the band-type CVT.

A fuel reformer is a device that converts heavier fuels into lighter compounds without fully combusting the fuel. A fuel reformer can be a catalytic reformer or a plasma reformer. Preferably, the reformer 30 is a partial oxidation catalytic reformer comprising a steam reforming catalyst. Examples of reformer catalysts include precious metals, such as Pt, Pd, or Ru, and oxides of Al, Mg, and Ni, the later group being typically combined with one or more of CaO, K₂O, and a rare earth metal such as Ce to increase activity. A reformer is preferably small in size as compared to an oxidation catalyst or a three-way catalyst designed to perform its primary functions at temperatures below 450° C. The reformer 30 is generally operative at temperatures from about 450 to about 1100° C.

The thermal mass 31 can be an inert substrate, such as a monolith without any catalyst coating.

The LNT 32 can comprise any suitable NOx-adsorbing material. Examples of NOx adsorbing materials include oxides, carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr, and Ba or alkali metals such as K or Cs. Further examples of NOx-adsorbing materials include molecular sieves, such as zeolites, alumina, silica, and activated carbon. Still further examples include metal phosphates, such as phosphates of titanium and zirconium. Generally, the NOx-adsorbing material is an alkaline earth oxide. The absorbent is typically combined with a binder and applied as a coating over an inert substrate.

The LNT 32 also comprises a catalyst for the reduction of NOx in a reducing environment. The catalyst can be, for example, one or more transition metals, such as Au, Ag, and Cu, group VIII metals, such as Pt, Rh, Pd, Ru, Ni, and Co, Cr, or Mo. A typical catalyst includes Pt and Rh. Precious metal catalysts also facilitate the adsorbent function of alkaline earth oxide absorbers.

Adsorbents and catalysts according to the present invention are generally adapted for use in vehicle exhaust systems. Vehicle exhaust systems create restriction on weight, dimensions, and durability. For example, a NOx adsorbent bed for a vehicle exhaust systems must be reasonably resistant to degradation under the vibrations encountered during vehicle operation.

The ammonia-SCR catalyst 33 is a catalyst effective to catalyze reactions between NOx and NH₃ to reduce NOx to N₂ in lean exhaust. Examples of SCR catalysts include oxides of metals such as Cu, Zn, V, Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Rd, Mo, W, and Ce, zeolites, such as ZSM-5 or ZSM-11, substituted with metal ions such as cations of Cu, Co, Ag, Zn, or Pt, and activated carbon. Preferably, the ammonia-SCR catalyst 33 is designed to tolerate temperatures required to desulfate the LNT 32.

Although not illustrated in any of the figures, a clean-up catalyst can be placed downstream of the other aftertreatment device. A clean-up catalyst is preferably functional to oxidize unburned hydrocarbons from the engine 21, unused reductants, and any H₂S released from the NOx absorber-catalyst 32 and not oxidized by the ammonia-SCR catalyst 33. Any suitable oxidation catalyst can be used. To allow the clean-up catalyst to function under rich conditions, the catalyst may include an oxygen-storing component, such as ceria. Removal of H₂S, where required, may be facilitated by one or more additional components such as NiO, Fe₂O₃, MnO₂, CoO, and CrO₂.

The invention as delineated by the following claims has been shown and/or described in terms of certain concepts, components, and features. While a particular component or feature may have been disclosed herein with respect to only one of several concepts or examples or in both broad and narrow terms, the components or features in their broad or narrow conceptions may be combined with one or more other components or features in their broad or narrow conceptions wherein such a combination would be recognized as logical by one of ordinary skill in the art. Also, this one specification may describe more than one invention and the following claims do not necessarily encompass every concept, aspect, embodiment, or example described herein. 

1. A method of operating a power generation system, comprising: operating a diesel engine to produce an exhaust containing NOx; passing the exhaust through an exhaust line containing a fuel reformer; passing the exhaust through a LNT that adsorbs and stores NOx from the exhaust under lean conditions; generating a control signal to regenerate the LNT to remove stored NOx; in a sequence of events trigged by the control signal, in a rich phase, providing fuel to the exhaust at a rate that leaves the exhaust rich, whereby the fuel reformer produces reformate-containing exhaust; passing the reformate-containing exhaust through a LNT to regenerate the LNT and reduce the adsorbed NOx; in the sequence of events trigged by the control signal, setting a throttle valve position limit based on a state of the diesel engine; and controlling an engine intake air throttle valve position to reduce the exhaust oxygen concentration; wherein the engine intake air throttle valve position is controlled to approach a setpoint while not exceeding the throttle valve position limit.
 2. The method of claim 1, further comprising: in the sequence of events trigged by the control signal, in a lean phase prior to the rich phase, warming the fuel reformer by providing fuel to the exhaust at a rate that leaves the exhaust lean, whereby fuel combustion in the exhaust line heats the reformer; wherein the reformer catalyzes steam reforming reactions during the rich phase.
 3. The method of claim 1, wherein the throttle valve position limit is set based at least in part on an exhaust oxygen concentration measured while the valve position is fully open.
 4. The method of claim 1, wherein the setpoint is determined based at least in part on an engine load.
 5. The method of claim 1, wherein the throttle valve position limit depends on the setpoint.
 6. The method of claim 5, wherein the limit depends on the setpoint in a manner that allows the throttle valve to close to a greater degree when the setpoint is higher as compared to when the setpoint is lower.
 7. The method of claim 1, wherein controlling the engine intake air throttle valve position comprises integrating an error and the integration ceases when the valve position reaches the limit.
 8. The method of claim 1, wherein controlling the engine intake air throttle valve position is accomplished with feedback based on a delay free estimate developed from a measured exhaust condition.
 9. A vehicle on-board computer method for controlling a position of a diesel engine intake air throttle valve, comprising: setting a software limit on the throttle valve position, wherein the limit is a value that is variable in a manner relating to one or more of engine speed and engine load; controlling a variable selected from the group consisting of exhaust oxygen concentration, exhaust oxygen flow rate, and exhaust air-fuel ratio to approach a setpoint value by adjusting the intake air throttle valve position without passing the limit.
 10. The method of claim 9, wherein the limit is set based at least in part on an exhaust oxygen concentration measured while the valve position is fully open.
 11. The method of claim 9, wherein the setpoint is determined based at least in part on the engine load.
 12. The method of claim 9, wherein the limit is made to depend on the setpoint.
 13. The method of claim 12, wherein the limit is made to depend on the setpoint in a manner that allows the throttle valve to close to a greater degree when the setpoint is higher as compared to when the setpoint is lower.
 14. The method of claim 9, wherein the control comprises integrating an error and the integration ceases when the valve position reaches the limit.
 15. The method of claim 9, wherein the control comprises feedback based on a delay free estimate developed from a measured exhaust condition.
 16. The method of claim 9, wherein the software limit is set as part of a sequence of steps carried out to regenerate a LNT.
 17. The method of claim 9, wherein the software limit is set as part of a sequence of steps carried out to regenerate a DPF.
 18. A power generation system comprising: a diesel engine operative to take in fuel from a fuel supply and air from an air supply and to produce an exhaust containing NOx; an engine intake air throttle valve configured to throttle the air supply; an exhaust line configured to receive the exhaust; a LNT configured in the exhaust line and operative to adsorb and store NOx from lean exhaust and to reduce the stored NOx and be regenerated when provided with rich reductant-containing exhaust; and a controller configured to control a position of the engine intake air throttle valve and to control a provision of reductant to the exhaust; wherein the controller is operative to regenerate the LNT by controlling the position of the engine intake air throttle valve and a rate of provision or reductant to the exhaust; the controller is configured to control the engine intake air throttle valve position during LNT regeneration to approach an exhaust oxygen content to a setpoint; and the controller is also configured not to adjust the valve position beyond a limit that is set dynamically by the controller at each regeneration.
 19. The system of claim 18, further comprising: a fuel reformer operative to produce reformate and configured to provide the reformate to the exhaust; wherein the reductant is reformate;
 20. The system of claim 19, wherein the fuel reformer is configured in the exhaust line upstream of the LNT.
 21. The system of claim 20, wherein the fuel reformer is adapted to catalyze steam reforming.
 22. The system of claim 21, wherein the controller is configured to regenerate the LNT by heating the fuel reformer under lean exhaust conditions prior to making the exhaust rich.
 23. The system of claim 18, wherein the controller is configured to set the throttle valve position limit based at least in part on a measured exhaust oxygen concentration.
 24. The system of claim 23, wherein the controller is configured to set the throttle valve position limit based at least in part on an exhaust oxygen concentration measured while the throttle valve is fully open.
 25. The system of claim 18, wherein the controller is configured to adjust the setpoint based on an engine load.
 26. The system of claim 18, wherein the controller is configured to set the throttle valve position limit based in part on the setpoint.
 27. The system of claim 26, wherein the controller sets the limit in a manner that allows the throttle valve to close to a greater degree when the setpoint is higher as compared to when the setpoint is lower.
 28. The system of claim 18, wherein the controller is configured to integrate an error between the setpoint and a feedback quantity and is configured to cease the integration when the valve position reaches the limit.
 29. The system of claim 18, wherein the controller is configured to control the valve position based on a delay-free estimate of the current exhaust oxygen concentration.
 30. The system of claim 18, wherein the controller is part of a set of controls in which the engine is controlled independently of the intake air throttle control.
 31. The system of claim 18, wherein the system is the product of combining an engine designed and manufactured without an intake air throttle valve with an exhaust aftertreatment system and an intake air throttle valve. 